Using the joint NASA/ESA/CSA James Webb Space Telescope, a group of astronomers has directly imaged three dusty disks — the outermost being an asteroid belt — surrounding a young star. The outermost of the disks was first discovered by NASA’s Infrared Astronomical Satellite in 1983 and has since been imaged in detail by the Hubble Space Telescope, Herschel Space Observatory, and the Atacama large millimeter/submillimeter array (ALMA). However, none of the images from Hubble, Herschel, or ALMA allowed scientists to determine the internal structure of the disk or the existence of additional disks — until now.
With Webb operational at the second Sun-Earth Lagrange point, scientists were eager to use its extremely sensitive infrared instruments to investigate the disk in infrared. When they finally got their hands on the observatory, they initially set out to just investigate the outermost disk surrounding the star. The results from Webb not only gave the team a look into the interior structure of the disk but also allowed them to discover and characterize two additional disks located closer to the star.
The young star that plays host to these dusty disks is Fomalhaut — a hot, bright star located in the southern Piscis Austrinus constellation approximately 25 light-years away from Earth. The three disks seen around Fomalhaut, called “debris disks,” were created from collisions between large bodies of gas, rock, and other cosmic material. These types of disks are commonly found around young stars and serve as places for left-over material from the formation of the star to clump together and potentially form planets or asteroids.
Webb has a new achievement under its (asteroid) belt!
This image is our first infrared look at an asteroid belt outside our solar system. Webb reveals there are actually 3 belts, including 2 never-before-seen inner belts, around the star of Fomalhaut: https://t.co/pDQKAh4hDA pic.twitter.com/GWed3Bbu4m
— NASA Webb Telescope (@NASAWebb) May 8, 2023
“I would describe Fomalhaut as the archetype of debris disks found elsewhere in our galaxy because it has components similar to those we have in our own planetary system. By looking at the patterns in these rings, we can actually start to make a little sketch of what a planetary system ought to look like – If we could actually take a deep enough picture to see the suspected planets,” said lead author András Gáspár of the University of Arizona in Tucson, Arizona.
As mentioned, before Webb’s latest observations of Fomalhaut, scientists had no idea that the two inner disks existed. The extreme sensitivity of Webb’s mid-infrared instrument (MIRI) to mid-infrared light allowed teams to see further into Fomalhaut’s planetary system, as much of the dust that makes up the three disks is too hot to be seen in visible light.
“Where Webb really excels is that we’re able to physically resolve the thermal glow from dust in those inner regions. So you can see inner belts that we could never see before,” said Schuyler Wolff of the University of Arizona.
“With Hubble and ALMA, we were able to image a bunch of Kuiper Belt analogs, and we’ve learned loads about how outer disks form and evolve. But we need Webb to allow us to image a dozen or so asteroid belts elsewhere. We can learn just as much about the inner warm regions of these disks as Hubble and ALMA taught us about the colder outer regions,” Wolff said.
Webb’s images didn’t only highlight the structure of the outermost ring and the inner two disks — it also allowed teams to measure the sizes of the disks and compare the characteristics of the disks to that of our own asteroid and Kuiper belts.
Webb found that the outermost of the three disks extends 23 billion kilometers from Fomalhaut, which measures to be about 150 times the distance of Earth from the Sun, and is approximately twice the size of our solar system’s Kuiper Belt — the massive disk of dust, gas, and small icy bodies that orbit the Sun beyond the orbit of Neptune. What’s more, Webb’s results showed that the disks are significantly more complex than our solar system’s main disks (the asteroid and Kuiper belts).
However, how were Fomalhaut’s disks sculpted and carved into what we see in Webb’s observations?
Astronomers believe that the three main disks of Fomalhaut were likely carved out by the gravitational forces of planets orbiting Fomalhaut. A similar phenomenon occurs in our solar system, as Jupiter shapes the outer asteroid belt while Neptune shapes the inner Kuiper belt. Given the small size of the planets relative to Fomalhaut and the disks, the planets can’t be seen in the Webb observations.
“We definitely didn’t expect the more complex structure with the second intermediate belt and then the broader asteroid belt. That structure is very exciting because any time an astronomer sees a gap and rings in a disk, they say, ‘There could be an embedded planet shaping the rings!’,” said Wolff.
As Webb continues to image other star systems, scientists will gain a better understanding of how planets shape the internal structures of star systems, like debris disks.
“The belts around Fomalhaut are kind of a mystery novel: where are the planets? I think it’s not a very big leap to say there’s probably a really interesting planetary system around the star,” said George Rieke, a member of the study and the U.S. science lead for MIRI.
Also seen in the Webb observations was what Gáspár et al. are calling “the great dust cloud.” Seen within the right side of the outermost disk, the team believes that the cloud could represent the collision between two young planetary bodies within the disk. This “dust cloud” is, interestingly, not the first time an object like this has been spotted within Fomalhaut’s outer disk. A similar object was spotted by Hubble in 2008 and was suspected to be a planet in the process of forming within the disk. Later observations of the cloud in 2014 showed it had disappeared, and scientists interpreted the cloud seen in 2008 as a collision between two bodies.
(Lead image: Webb’s image of Fomalhaut’s three dusty disks. Credit: NASA/ESA/CSA/A. Gáspár (University of Arizona)/A. Pagan (STScI))